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Chemical and Physical changes are accompanied by changes in energy Energy moves in the form of HEAT (q) and WORK (w) Get a feel for heat and work Get a feel for energy units
What are we going to learn today?
EVERY change (physical or chemical) is accompanied by a change in energy The First Law of Thermodynamics
All energy is conserved, it can not be created or destroyed. It is simply transferred from one form to another.
Energy
When I think of types of energy, I think: a) KE and PE are the same as heat & work b) PE and KE are the same as heat & work c) PE and KE are the only two forms of energy d) Heat and work are the only two forms of energy
KE = Kinetic Energy PE = Potential Energy
POLL: iCLICKER QUESTION
What is Energy? Potential Energy (PE) energy due to position or composition Kinetic Energy (KE) energy of the motion of an object or particle Units: J
Energy Definitions
How does Energy move? Heat (q) transfer of energy from a hotter body to a colder body (NOTE: This is not temperature) Work (w) transfer of energy via applied force over distance Units: J
Energy Definitions
Drop a ball to the floor. Where did the Energy go? Please describe the Energy of the Ball. Unfortunately, a chemical change is a little more difficult to visualize.
Demonstration
System and State
A system is the part of the universe on which we want to focus our attention. The surroundings are everything else The universe is the system and the surroundings
Universe = system + surroundings We also describe chemical changes with beginning and end states A change in a chemical reaction is described as
Δ State = Stateend – Statebeginning
Demonstrations
Physical Change (doing work) Physical Change (absorbing heat) Chemical Change (releasing heat)
Thermodynamics
Chemists care about understanding and quantifying the amount of energy that moves into or out of a system upon a change. Energy moves in the form of heat (q) and work (w)
Heat (q)
Heat is energy transferred as a result of temperature difference. Temperature is a property that reflects the random motions of the particles in a particular substance. q is the symbol used to indicate energy changed by receiving or losing heat
Heat (q)
Sign Notation of q
+ when system receives heat from surroundings
- when system gives heat to the surroundings
System
Surr to Sys + q
Sys to Surr
- q
Work is transfer of energy via applied force over distance w = Force x distance Chemists are mostly concerned with PV work P = pressure and V = volume w = - PΔV
Expansion work is an expansion against an external force
w = - PexΔV Can you think of an example of PV work?
Work (w)
Expansion Work against Constant External Pressure
• System: Piston, area A:
– pushes against external (surrounding) pressure P • Pressure = force/unit area = F/A
– experiences opposing force
F = Pext A
– System
– w should be _________________
Pex MUST BE CONSTANT!
Expansion Work against Constant External Pressure
• Move piston out a distance ∆h:
– work = f x d
– │work │ = │ Pext A ∆h│
– What is A ∆h?
– │w │ = │ Pext ∆V │
– ∆V is _________
– w should be ___________
• w = - Pext∆V
Pex MUST BE CONSTANT!
Work (w)
Sign Notation of w
+ work done ON the system BY the surroundings
- work done BY the system ON the surroundings
System
BY Surr ON Sys + w
BY Sys ON Surr
- w
Today you saw a demonstration in which a lid popped off a container that contained a piece of CO2(s). In this situation: a) Heat was transferred into the system, Work was done by the system. b) Heat was transferred out of the system, Work was done by the
system. c) Heat was transferred into the system, Work was done on the
system. d) Heat was transferred out of the system, Work was done on the
system. e) Heat was not transferred, only Work was done by the system.
POLL: iCLICKER QUESTION
Today you saw a demonstration in which a lid popped off a container that contained a piece of CO2(s). In this situation: a) +q, +w b) +q, -w c) -q, +w d) -q, -w
POLL: iCLICKER QUESTION
The gases in the four cylinders of an automobile engine expand from 0.22 L to 2.2 L during one ignition cycle. Assuming that the gear train maintains a steady pressure of 9.60 atm on the gases, how much work can the engine do in one cycle? (1 L•atm = 101.325 J) A. 19 J B. -19 J C. 1945 J D. -1945 J
POLL: iCLICKER QUESTION
Summary
SYSTEM w < 0
w > 0
q < 0
q > 0
Heat released by system
Work done by system
Heat absorbed by system
Work done on system
Surroundings
Universe
First Law of Thermodynamics Energy can not be created or destroyed Law of Conservation of Energy Universe = System + Surroundings Internal Energy (U)
We can calculate the change in internal energy (ΔU) of the system by combining heat and work
ΔU = q + w
Thermodynamics
A system absorbs 72 J of heat while 35 J of work is done on it. Calculate ΔU. A. -107 J B. 107 J C. -37 J D. 37 J E. 0 J
POLL: iCLICKER QUESTION
A system was heated by using 600 J of heat, yet it was found that the internal energy decreased by 150 J . Calculate w. A. 450 J B. -450 J C. 750 J D. -750 J
POLL: iCLICKER QUESTION
In the last question, was work done on the system or by the system? A. On B. By
(A system was heated by using 600 J of heat, yet it was found that the internal energy decreased by 150 J . Calculate w. )
POLL: iCLICKER QUESTION
The air inside a balloon is heated, allowing for the balloon to fill to its full capacity. The volume of the balloon changes from 4.0x106 L to 4.5x106 L by the addition of 1.3x108 J of energy as heat. Assuming the balloon expands against a constant pressure of 1.0 atm, calculate the ΔE for the process. (1 L•atm = 101.325 J)
A. 1.2 x 108 J B. -1.2 x 108 J C. 7.9 x 107 J D. -7.9 x 107 J
POLL: iCLICKER QUESTION
First Law of Thermodynamics: Energy is conserved in universe- Law of Conservation of Energy ΔU = q + w Heat = movement of energy from hotter body to colder body Work = force x distance = – PΔV Sign convention important “-” work done by system “+” work done on the system
What have we learned today?
Learning Outcomes
Understand the concept of the energy units: calorie, kilocalorie and kilojoule Understand the concept of internal energy, heat and work State and use the equation for change in internal energy, ΔU Understand all sign conventions in for all the thermodynamic concepts Calculate w for expansion or compression against a constant pressure.
Important Information
LM26 & LM27 POSTED No HW
LM27 contains a link to Unit 4 worksheets
You should complete the worksheets associated with properties and change and the first law
Dr. Sparks does not have office hours today.
No UGLA office hours today.
Exam Grades will be posted as soon as possible!